This research project includes an investigation of the physiologic control mechanisms regulating the activation and functions of the first component of human complement (C1). Additionally, studies are proposed to elucidate the molecular mechanism of C1 activation, and to define the role of antibody in this process. We will investigate the influence of immune complex size and antibody class on ability to activate complement, in particular, in the presence of the regulatory protein C1-Inhibitor (C1-In). We will also study the mechanism by which certain immune complexes escape control by C1-In. Do immune complex surface structures, as carbohydrate, restrict C1-In interaction with C1? Furthermore, we will determine why immune complexes can readily turn over C1 in a purified system but not in NHS. The roles of C3b and C4b will be considered, and any new serum factors will be characterized, thereby enhancing our understanding of physiologic regulation of Fc function. The effect of protease nexin (an inhibitor released from fibroblasts) on C1 will be studied. Additionally, serum factors, as metal ions and lipoproteins, that may potentiate C1-In function, will be examined. These studies are relevant for a better understanding of hereditary angioedema and immune complex diseases. Methodology will be developed to characterize subtle structural changes that accompany C1 activation. Conformational changes in the region of metal ion binding sites of C1 will be studied using lanthenide metal fluorescence. Conformational changes in the region of C1 active sites will be studied by ligand quenching of intrinsic fluorescence. Changes in surface hydrophobicity of C1 with activation will be characterized using fluorophores. These studies will determine if immune complexes induce conformational changes in C1 to initiate C1 activation. We will also perform experiments ot support an alternative (or complementary) hypothesis: antibody may actually play a passive role in C1 activation by providing a site of assembly of a stable macromolecular C1 complex, thus potentiating the inherent ability of C1 to autoactivate. We will determine if the efficiency of C1 activation is dependent on the strength of association between C1q and C1r2s2. Finally, is the interaction of C1r2s2 with activator critical, and if so, is antibody carbohydrate important?